Our data support the hypothesis that endogenous estrogen (E2) or a chemoprotective diet containing estrogenic compounds reduce the risk of colon tumor development. This is consistent with human studies demonstrating that exposure to estrogen in the colon protects against malignant transformation. Although there are concerns related to estrogen therapy, dietary phytoestrogens may provide a suitable alternative. Indeed, epidemiological data suggest that soy, containing phytoestrogens such as genistein (Gen), reduces colon cancer risk. With respect to putative targets, adult somatic stem cells of the colon are of particular interest because they sustain self-renewal and are target cells for cancer initiating mutations. Recent evidence indicates that normal intestinal stem/progenitor cells can initiate colon tumorigenesis and drive cancer progression towards metastasis. Unfortunately, to date, the effects of endogenous and dietary estrogens on intestinal stem cell signaling has not been determined. Since estrogenic compounds can impact genes/mediators that regulate the colon stem cell niche and tumor evolution, e.g. p53 signature, our overall goal is to further elucidate how E2 and dietary Gen modulate colonic stem cells at distinct stages of malignant transformation. Based on our experimental findings, we propose two specific aims to determine how endogenous (E2) and dietary estrogen (Gen) impact multipotent stem cells in the colon. Aim 1 will use highly novel stem cell specific Lgr5-LacZ and Lgr5-EGFP mice to quantify the number and spatio-temporal location of stem cells, DNA damage and targeted apoptosis in the colonic crypt at the initiation and tumor stages of colon carcinogenesis following exposure to endogenous and diet-derived estrogens. Aim 2 will use Lgr5-stem cell targeted p53 null mice (Lgr5-EGFP-IRES-creERT2 x p53flox/flox) to define the molecular role of p53 in mediating the effects of E2 and Gen in intestinal stem cells. The proposed studies are highly innovative and will explore the physiological actions of dietary estrogenic compounds on multipotent stem cells in the colon.